The hydrogen exchange of coals with water was investigated using a tritium tracer method to estimate the mobility of hydrogen in coals. The reactions at several temperatures between 50 and 300 degrees C were carried out using a glass batch reactor and a pulse flow reactor, respectively. At lower temperatures, the ratio of hydrogen exchange of coal with water increased with a decrease in rank of coals and tended to change with respect to the content of functional groups such as hydroxyl group, thiol, amino group, and carboxylic acid in coal. From the results obtained from the hydrogen exchange of model compounds of the functional groups present in coal, it is proposed that the hydrogen only in the functional group was exchangeable at lower temperature while the hydrogen in aromatic ring substituted by functional groups also became exchangeable at 300 degrees C. It was found that the use of the pulse flow reactor as well as the glass batch reactor was very useful facile and convenient methods to trace the hydrogen exchange between coal and water.